12 The MmpL Protein Family

Affiliations: 1: Department of Microbiology and Immunology, University of California, San Francisco, San Francisco, CA 94143-2200;
2: Department of Microbiology and Immunology, University of California, San Francisco, San Francisco, CA 94143-2200;
3: Department of Microbiology and Immunology, University of California, San Francisco, San Francisco, CA 94143-2200

The diverse array of Mycobacterium tuberculosis lipids, many of which are critical for virulence and mediate interactions with host cells, exist primarily on the outermost surface of the bacterium. Fourteen mmpL genes have been identified by analysis of the two completed M. tuberculosis genomic sequences. These genes encode for large, multitransmembrane containing proteins and were thus given the moniker MmpL (for mycobacterial membrane protein, large). Initial models of MmpL function were based on the RND transporters from gram-negative bacteria. MmpL7 was the first MmpL family member to be studied and was identified as required for virulence and bacterial growth in vivo in a mouse model of M. tuberculosis infection. Mutants in mmpL8 were also isolated in a signature- tagged mutagenesis screen, and MmpL8 is indispensable for growth and virulence in mice. First, mmpL4 transcription is induced, albeit modestly, during infection of activated macrophages. The eukaryotic larger resistance, nodulation, and division (RND-family member, Niemann-Pick C1 (NPC1) utilizes the proton motive force to translocate cholesterol across the cell membrane and it is widely thought that RND family members use proton antiport or symport as a mechanism of facilitated transport of their substrates. The Lol system functions to export lipoproteins across the periplasm of Escherichia coli to the inner leaflet of the outer membrane. LolCDE bind specifically to lipoproteins destined for the outer membrane and export them across the cell membrane, where they bind the periplasmic shuttle LolA.

Phylogenetic tree of MmpL proteins from sequenced mycobacterial species. The MmpLs from M. tuberculosis are shown in bold. Most of the M. bovis orthologues of the M. tuberculosis MmpLs are omitted because they are identical in sequence except Mb1177, which has a frameshift mutation in M. tuberculosis, and Mb1583, which is a full-length version of MmpL6. Mb, M. bovis; ML, M. leprae; MAP, M. avium; MM, M. marinum; MUL, M. ulcerans; MSMEG, M. smegmatis.

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Figure 1.

Phylogenetic tree of MmpL proteins from sequenced mycobacterial species. The MmpLs from M. tuberculosis are shown in bold. Most of the M. bovis orthologues of the M. tuberculosis MmpLs are omitted because they are identical in sequence except Mb1177, which has a frameshift mutation in M. tuberculosis, and Mb1583, which is a full-length version of MmpL6. Mb, M. bovis; ML, M. leprae; MAP, M. avium; MM, M. marinum; MUL, M. ulcerans; MSMEG, M. smegmatis.

Gene organization of MmpLs. Genes predicted to be involved in lipid transport, including the mmpL and mmpS genes, the lppQ and lppX genes, and the drrABC operon are shown as white arrows. Genes predicted to be involved in lipid biosynthesis and metabolism are shown in black.

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Figure 3.

Gene organization of MmpLs. Genes predicted to be involved in lipid transport, including the mmpL and mmpS genes, the lppQ and lppX genes, and the drrABC operon are shown as white arrows. Genes predicted to be involved in lipid biosynthesis and metabolism are shown in black.

Model of MmpL secretion. The proposed models for PDIM and SL-1 secretion through MmpL7 and MmpL8, respectively, are shown. PpsA-E and Mas are polyketide synthases that extend straight chain fatty acids to phthiocerol and mycocerosic acid, respectively (Azad et al., 1997; Azad et al., 1996; Trivedi et al., 2005). The enzymes FadD26 and FadD28 are thought to be AMP ligases that activate straight-chain fatty acids for transfer to the Pps and Mas enzymes (Trivedi et al., 2004). The thioesterase TesA is also required for the synthesis of PDIM and interacts with PpsE (Rao and Ranganathan, 2004). PapA5 is able to catalyze the esterification of mycocerosic acids to phthiocerol to form PDIM (Onwueme et al., 2004). MmpL7 and DrrABC are required to transport PDIM across the cell membrane (Camacho et al., 1999; Cox et al., 1999). Finally, LppX is a signal sequence containing protein that is thought to transport PDIM across the periplasm to the cell wall (Sulzenbacher et al., 2006). Two models for SL-1 secretion are shown. Pks2 is required for the synthesis of SL1278 (Sirakova et al., 2001). In model A, MmpL8 recruits an as yet unidentified biosynthetic factor to complete the synthesis of SL-1 from SL1278, before transport across the cell membrane (Jain and Cox, 2005). In model B, MmpL8 exports SL1278 across the cell membrane, after which it is converted to SL-1 by an as yet unidentified enzyme (Converse et al., 2003). CM, cytoplasmic membrane; PG, peptidoglycan; mAG, mycolyl-arabinogalactan. (See the color insert for the color version of this figure.)

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Figure 4.

Model of MmpL secretion. The proposed models for PDIM and SL-1 secretion through MmpL7 and MmpL8, respectively, are shown. PpsA-E and Mas are polyketide synthases that extend straight chain fatty acids to phthiocerol and mycocerosic acid, respectively (Azad et al., 1997; Azad et al., 1996; Trivedi et al., 2005). The enzymes FadD26 and FadD28 are thought to be AMP ligases that activate straight-chain fatty acids for transfer to the Pps and Mas enzymes (Trivedi et al., 2004). The thioesterase TesA is also required for the synthesis of PDIM and interacts with PpsE (Rao and Ranganathan, 2004). PapA5 is able to catalyze the esterification of mycocerosic acids to phthiocerol to form PDIM (Onwueme et al., 2004). MmpL7 and DrrABC are required to transport PDIM across the cell membrane (Camacho et al., 1999; Cox et al., 1999). Finally, LppX is a signal sequence containing protein that is thought to transport PDIM across the periplasm to the cell wall (Sulzenbacher et al., 2006). Two models for SL-1 secretion are shown. Pks2 is required for the synthesis of SL1278 (Sirakova et al., 2001). In model A, MmpL8 recruits an as yet unidentified biosynthetic factor to complete the synthesis of SL-1 from SL1278, before transport across the cell membrane (Jain and Cox, 2005). In model B, MmpL8 exports SL1278 across the cell membrane, after which it is converted to SL-1 by an as yet unidentified enzyme (Converse et al., 2003). CM, cytoplasmic membrane; PG, peptidoglycan; mAG, mycolyl-arabinogalactan. (See the color insert for the color version of this figure.)